1. General Field
My invention is in the field of fiber optics, and also in the field of burglar alarms or intrusion sensors. In particular my invention relates to use of an optic fiber embedded in a transparent barrier or premises window, to generate an alarm signal when the barrier or window is breached.
2. Prior Art
Earliest burglar-alarm sensors were purely mechanical, involving linkages actuated by doors and windows, or cord stretched across apertures, to operate alarm bells. Later it became feasible to substitute an electrical circuit between the door-actuated "trip" or the stretched cord and an electrically-driven annunciator. Electrical systems in combination with glass windows naturally gave rise to the metallic-tape technique for store windows and sometimes display cases. With the advent of electric lights and photocells the light-beam system became popular for sensing entries, and secondarily for sensing unwanted intrusions.
All of the systems mentioned so far have two principal disadvantages:
(1) Coverage of protection is relatively gross in a geometric sense. That is, most actual systems of these types leave fairly extensive areas unprotected against sophisticated intruders able to cut their way into the guarded area at selected points. For example, a glass door which is protected by a switch that is operated when the door opens, and also protected by metallic tape around the edge of the glass, is breached by cutting the glass at a point not reached by the tape, and reaching inside to short-circuit the door switch, whereupon the door can be opened without sounding the alarm. Alternatively a thief can short-circuit the metallic tape, whereupon the glass can be breached or completely removed, likewise without sounding the alarm. If the tape were applied at such narrow intervals as to prevent such techniques, the result would be to defeat the purpose of using glass: much of the panel which is desired to be transparent would be obstructed by the metallic tape. The gross character of the protection is compounded by the fact that a prospective intruder can readily localize the protected areas, and apply his efforts elsewhere, because the tape, switches, strings, and optical beams are all relatively easy to see.
(2) Some of the same systems are also objectionable on esthetic grounds. The metallic tape, for instance, is quite conspicuous and in many instances may detract from the intended visual effect of a valuable display.
Some more-sophisticated systems operate by responding to sounds or other vibrations produced by intruders. These techniques usually obviate the two types of disadvantages mentioned above, but have two of their own:
(1) They are inappropriate to certain kinds of situations, in which the anticipated ambient noise or vibration level is normally high. For instance, in a busy museum setting the noise of conversation and moving visitors may be as high as, or higher than, the sound of an intrusion into a display case. Likewise, in a neighborhood where street noise and vibration levels are high even late at night, a sonic or vibration-sensitive system possibly cannot be set sensitive enough to respond to intrusion without its generating nearly constant false alarms due to the outdoors ambient noise or vibration.
(2) These systems are also nonspecific in another way--they tend to be tripped by inconsequential indoors events, such as an office cat knocking something over at night, or a watchdog barking at innocuous activity outdoors.
The most elaborate systems involve setting up fields--electric, electromagnetic, sonic, etc--in the space to be protected, and detecting disturbance of the fields by intruders. These systems represent a considerable improvement as to specificity of response, as compared with the sonic or vibration detectors, but are objectionably expensive. For some applications they are also too bulky, and they can be "temperamental."
My invention is directed to providing reliable and consistent protection of premises windows or transparent barriers, on as fine a geometrical grid as desired, in a way which is not merely inconspicuous but actually imperceptible, and at reasonable cost.
The invention is amenable to nonplanar, elaborately shaped display enclosures, such as transparent cylinders, domes, or even spheroids, as well as cubes, multifaced closed figures, and irregularly shaped enclosures.
I know of no prior art which approaches the concepts disclosed and claimed herein.